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) Dear Governor Askew:
The Bureau of Geology of the Division of Resource Management,
Florida Department of Natural Resources, is pleased to publish as
its Report of Investigations No. 82, a study, "Hydrologic Effects
of the Tampa Bypass Canal System," by Louis H. Motz of the U. S.
Geological Survey.
The Tampa Bypass Canal is to divert flood waters of the Hills-
borough River above the cities of Tampa and Temple Terrace and
into McKay Bay. The canal system will breach the underlying
artesian Floridan aquifer in places causing drawdowns of a wide
area. This study is to access the hydrologic effects of the canal sys-
tem and to determine means of reducing the adverse hydrologic
effects of the canal system.
We believe this approach to problem-solving to be most impor-
tant in providing protection to flood-prone areas while minimizing
the impact on the hydrologic regimen.

Respectfully yours,

Charles W. Hendry, Jr., Chief
Bureau of Geology

Completed manuscript received
May 7, 1975
Printed for the
Florida Department of Natural Resources
Division of Resource Management
Bureau of Geology
Tallahassee
1976

Figure Page
1. Map showing route of Tampa Bypass Canal System in the
T am pa area .... .................. ............................................................................................................ 3
2. Map showing canals and structures of Tampa Bypass
C ana l S y stem .. ....................... .............................. .......................................................................... 5
3. Profiles showing pool-level altitudes along Tampa Bypass Canal
System from McKay Bay to Hillsborough River ................................................ 7
4. Profiles of the potentiometric surface of the Floridan aquifer
along the route of the canal in May 1972 and the pool levels in
the canal from S-160 to S-159, S-161, and S-153 ............................ ........... 9
5. Graph of mean monthly rainfall at Tampa, Florida, 1890-1970 ............ 10
6. Map showing location of hydrogeologic sections .............................................. 12
7. Hydrogeologic section along Canal 135 ................................................................... 13
8. Hydrogeologic section along Canal 136 and Canal 132............................... 14
9. Hydrogeologic section at Harney Flats ................................................................. 15
10. Map showing location of ground-water data-collection sites ....................... 19
11. Map showing potentiometric surface of the Floridan aquifer,
M ay 1972 ............................................................................ ........................................................... ..... 22
12. Graph showing hydrogeologic data for well P-SMC at Sixmile
C reek test site ....................................................................................................................................... 25
13. Graph showing hydrogeologic data for well P-ES in Harney Flats
near Eureka Springs ........ ............... ........... .............. .................... ............................... .......... 26
14. Hydrogeologic section at Harney Flats showing lowering of the
potentiometric surface of the Floridan aquifer that would result
from the canal system as presently designed ............................................................ 28
15-20 Maps showing:
15. Parts of the canal system treated as a line sink and boundaries
of dig ital m odel .1..... ............ .......................................... ................................................................ 31
16. Drawdown that would have occurred during the May 1972 low
water-level period due to the present design of C-135, C-136,
and C-132 if the canal had been built ................................................. ..................... 33
17. Potentiometric surface of the Floridan aquifer as it would have
occurred in May 1972 if C-135, C-136, and C-132 had been built............ 34
18. Drawdown that would have occurred during the May 1972 low
water-level period due to the modified design of C-135, C-136,
and C-132 with the additional structure near Buffalo Avenue if
the canal had been built .......................................................................................................... 35
19. Drawdown that would have occurred during the May 1972 low
water-level period due to the present design of C-135 and C-136
without C-132 if the canal had been built ............................................................... 36
20. Drawdown that would have occurred during the May 1972 low
water-level period due to the modified design of C-135 and C-136
without C-132 but with the additional structure near Buffalo
Avenue, if the canal had been built ......................................................................... 37
21. Graph showing area-drawdown relations for C-135, C-136, and
C-132 that would have occurred during the May 1972 low water-
level period if the canal had been built ........................................................................ 38

22. Graph showing area-drawdown relations for C-135 and C-136
that would have occurred during the May 1972 low water-level
period if the canal had been built ............................................................................. 39

TABLES

Table Page
1. Design details of the Tampa Bypass Canal System ................................... 6
2. Wells at which data were collected ...................... .......... 16,17,18
8. Analyses of water samples from the Floridan aquifer in the
area of the Tampa Bypass Canal System .................................................... 20,21
4. Discharge into the canal system from the Floridan aquifer that
would have occurred during the May 1972 low water-level period
if the canal had been built ............................................................ ..................... 40

CONVERSION FACTORS

Factors for converting English units to metric units are shown to
four significant figures. However, metric equivalents should be con-
verted only to the number of significant figures given for the values
of English units in this report.

Flood water of the Hillsborough River will be diverted at a point
upstream from areas of flood-plain encroachment in the cities of
Tampa and Temple Terrace into nearby McKay Bay by means of the
Tampa Bypass Canal System, which is being built by the U. S.
Army Corps of Engineers through an area east of Tampa. The canal
system will breach the underlying artesian Floridan aquifer in
places where the potentiometric surface of the aquifer is at or near
land surface, causing drawdowns over a wide area and diverting
flow from other parts of the hydrologic system. An investigation by
the U. S. Geological Survey in cooperation with the Corps of Engi-
neers in conjunction with the Southwest Florida Water Manage-
ment District was made to assess the hydrologic effects of the canal
system and to determine whether an additional water-level control
structure could reduce significantly the hydrologic effects of the
canal system. The additional structure would maintain the pool
level in the Eureka Springs and Harney Flats areas at a 5-foot
higher level than without the structure, reducing drawdown in and
discharge from the Floridan aquifer in these areas. The assessment
of hydrologic effects was based on water-level conditions of the
May 1972 low-water period. The assessment was made for a sea-
sonal low-water period, because the effect that the discharge into
the canal system will have on ground-water discharge at other
places in the hydrologic system and on flow in the Hillsborough
River will be most critical during the dry season.
The canal system, as presently designed, will lower the poten-
tiometric surface of the Floridan aquifer by 1 foot or more over an
area of about 92 square miles with C-132 (Thonotosassa Canal)
included in the system and about 48 square miles without C-132;
discharge from the aquifer into the canal system will be 22 million
gallons per day with C-132 and 15 million gallons per day without
C-132. The discharge into the canal system will cause a decrease
in the present (pre-canal system) discharge from the Floridan
aquifer to other parts of the hydrologic system. If another control

BUREAU OF GEOLOGY

structure is added to the canal system near Buffalo Avenue, the
area of drawdown (1 foot or more) of the potentiometric surface
will be less by 45 percent and the discharge less by 40 percent with
C-132 included in the system; without C-132, the area of drawdown
will be less by 50 percent and the discharge less by 45 percent. Be-
cause certain segments of the canal system were assumed to be in
direct hydraulic connection with the Floridan aquifer along their en-
tire length, an assumption which is valid for only part of their
length, the values represent a maximum condition.

INTRODUCTION
The U. S. Army Corps of Engineers (1961) has proposed a plan
of stream improvements and a system of canals, reservoirs, and
structures for water control and flood protection for the Four River
Basins area, which includes 6,000 mi' in central and southwest
peninsular Florida. The Tampa Bypass Canal System is part of this
plan. The canal system is designed to divert flood waters from the
Hillsborough River at a point upstream from areas of flood-plain
encroachment in the cities of Tampa and Temple Terrace and pass
the water through an area east of Tampa into nearby McKay Bay
(fig. 1).
The canal system will breach the underlying artesian Floridan
aquifer in several places. Concern was expressed by the Southwest
Florida Water Management District, the local project sponsor, that
the canal system may cause drainage from the aquifer and may
affect ground-water levels over a large area. The quality of the
ground water near the canal system also may be affected. There-
fore, in 1971, the U. S. Geological Survey in cooperation with the
Corps of Engineers in conjunction with the Southwest Florida
Water Management District began an investigation of the hydro-
logic effects of the canal system.

PURPOSE AND SCOPE
This report presents the results of a study begun in January
1972 to assess the hydrologic effects of the canal system and to
determine whether incorporating an additional water-level control
structure into the design of the canal system could reduce its hy-
drologic effects. Data obtained from published sources, from U. S.
Geological Survey and Corps of Engineers files, and from field in-
vestigations made as part of this study were used in predicting the
effects of the canal system. Aquifer tests were made to determine

REPORT OF INVESTIGATION NO. 82

870 860
I I

85 840 830 82 81 80
I I I L 310

-29

-280

- 270

-260

-250

Figure 1. Route of Tampa Bypass Canal System in the Tampa area.

the hydrologic properties of the underlying aquifer, and geophysical
methods were used to determine the principal water-bearing zones
in the upper part of the aquifer. A digital model of the ground-
water system was used to predict the amount of discharge into the
canal system from the aquifer and the extent of drawdown due to
the canal system. Also data were collected to provide a reference
from which to determine later how much the canal system has af-
fected water levels and water quality in the area.

BUREAU OF GEOLOGY

ACKNOWLEDGMENTS
Appreciation is expressed to Garald G. Parker, senior scientist
and chief hydrologist, Southwest Florida Water Management Dis-
trict, for his assistance during the initial phase of the study.
Thanks are given to Charles F. Dreves, Jr. and Andrew 0. Hobbs,
Corps of Engineers, for their aid in the exchange of information
needed for this investigation. The cooperation of local landowners
and Hillsborough County officials in granting access to existing
wells and in permitting construction of new wells is gratefully
acknowledged. Discussions with Glen L. Faulkner, U. S. Geological
Survey, were very helpful. Appreciation is also expressed to Ray-
mond A. Mularoni, Bobby D. Ramsey, and Robert L. Westly, who
helped collect the data used in this report.
The investigation was made and the report prepared under the
direct supervision of Joseph S. Rosenshein, subdistrict chief,
Tampa, and under the general supervision of Clyde S. Conover,
district chief, Tallahassee.
PREVIOUS INVESTIGATIONS
The geohydrology of the area in which the canal system is being
built (fig. 1) has been described by. several investigators. The U. S.
Army Corps of Engineers (1961)1 has prepared reports and un-
published design memoranda on many aspects of the Bypass Canal
System project, including the hydrology and geology. Cooke (1945),,
Carr and Alverson (1959), and Puri apd Vernon (1964) included
descriptions of the geology of the area. MacNeil (1949) and White
(1958) discussed aspects of the physiography and geomorphology
of parts of Florida, including the area near Tampa. Stringfield
(1936, 196-1, 1966) studied the hydrology of Florida, including the
Tampa area, and prepared maps of the potentiometric surface of
the principal artesian (Floridan) aquifer. Stewart and others
(1971) prepared a potentiometric map of the Floridan aquifer in
the Southwest Florida Water Management District area including
Tampa. Menke and other (1961) studied the water resources of
the Tampa area and Shattles (1965) reported on water quality in
the area.
CANAL SYSTEM
CANALS AND STRUCTURES
The Tampa Bypass Canal System will have a total length of
about 27 mi and will consist of three canal segments: the Tampa
Bypass Canal, the Harney Canal, and the Thonotosassa Canal (U.S.

REPORT OF INVESTIGATION NO. 82

Figure 2. Canals and structures of Tampa Bypass Canal System.

Army Corps of Engineers, 1961,) (fig. 2). The Tampa Bypass
Canal, or Canal 135 (C-135), will extend from McKay Bay north-
ward 14.0 mi to the Hillsborough River. From McKay Bay, C-135
will follow Palm River and Sixmile Creek 4.4 mi upstream to the
first water-level control structure, Structure 160 (S-160). From
S-160, the canal will follow Sixmile Creek drainage canal and will
pass through a low, swampy area known as Harney Flats. The next
water-level control structure, Structure 159 (S-159), will be 6.0 mi
upstream from S-160. From S-159, the canal will run northward
3.6 mi to Structure 155 (S-155) at the Hillsborough River.
The Harney Canal, or Canal 136 (C-136), will extend north-
westward from its confluence with C-135 in Harney Flats 1.7 mi
to the Hillsborough River. A water-level control structure, Struc-
ture 161 (S-161) will be built 0.3 mi from the river.
The third segment is the Thonotosassa Canal, or Canal 132

(C-132), which will extend northeastward from
C-135 in the Eureka Springs area 11.3 mi thro
sassa and along Flint Creek to the Hillsboroul

its confluence with
ugh Lake Thonoto-
rh River. A water-

level control structure, Structure 153 (S-153), will be built 3.7 mi
upstream from C-135, and another structure, Structure 154 (S-154),
will be built between Lake Thonotosassa and the Hillsborough River
1.2 mi from the river.
The spillway capacities of the control structures are based on
the occurrence of the maximum probable flood. The design dis-
charges for S-160 and S-159 are 26,700 and 12,000 cfs respectively.
At S-155, 22,800 cfs can be released to the downstream reach of
the Hillsborough River. The design discharge for S-161 is 4,000 cfs,
and the design discharges for S-153 and S-154 are 3,100 and 500 cfs
respectively.
'he canal system (table 1) is being constructed northward
from its downstream end at McKay Bay. At present (1973), S-160

and sections of C-135 along Palm River have been completed, and
the sections northward to Buffalo Avenue are under construction.
Design work and right-of-way acquisition are under way for the
rest of the canal system.
As presently (1973) designed, water levels in the canal system
will be maintained during nonflood periods by the structures at the
levels shown in figure 3. In C-135, the minimum pool levels will be
sea level from McKay Bay to S-160, 10 ft above msl (feet above
mean sea level) from S-160 to S-159, and 30 ft above msl from
S-159 to S-155. In C-136, the minimum pool levels will be 10 ft above
msl from C-135 to S-161, and 20 ft above msl upstream from S-161
to the Hillsborough River. In C-132, the minimum pool levels will
be 10 ft above msl from C-135 to S-153 and between 30 and 34 ft
above msl between S-153 and the Hillsborough River.

DESIGN MODIFICATIONS
As part of this study, the effects of adding another water-level
control structure near Buffalo Avenue (fig. 2) were investigated.
This control structure would maintain the pool level in the Eureka
Springs and Harney Flats areas at a 5-foot higher level than with-
out the structure. (fig. 4). This would result in less drawdown of
the potentiometric surface of the Floridan aquifer and less dis-
charge from the aquifer in these areas, reducing the hydrologic
effects. The present design of the canal system calls for a minimum
pool level to be maintained at 10 ft above mean sea level from S-160
northward to S-159 in C-135, to S-161 in C-136, and to S-153 in
C-132. With the additional structure the minimum pool level still
would be 10 ft above msl from S-160 to just north of Buffalo
Avenue but would be maintained at 15 ft above msl from just north
of Buffalo Avenue northward to S-159, S-161, and S-153.
The selection of the site near Buffalo Avenue for a possible
additional control structure was made by the Southwest Florida
Water Management District and the Corps of Engineers on the
basis of hydrologic and topographic factors. The pool level behind
the additional structure would be approximately equal to the May
1972 level of the potentiometric surface of the Floridan aquifer in
the vicinity of Buffalo Avenue. Also, the 15-foot level would be the
maximum practical level that could be maintained in the low-lying
Eureka Springs and Harney Flats areas without resorting to an
extensive system of levees.
In this study, the effects of adding this control structure near

Figure 4. Profiles of the potentiometric surface of the Floridan aquifer along
the route of the canal in May 1972 and the pool levels in the canal
from S-160 to S-159, S-161, and S-153.

Buffalo Avenue were considered for a canal system consisting of
C-185 and C-136 with C-132 (the Thonotosassa Canal) and also for
a system consisting of C-135 and C-136 without C-132, because
C-132 may not be constructed at the same time that the other seg-
ments of the canal system are constructed (oral commun., U. S.
Army Corps of Engineers, 1972).

DESCRIPTION OF AREA

CLIMATE

Tampa's climate (U. S. Dept. Commerce, 1972) is characterized
by a summer thundershower season occurring between a relatively
dry spring and fall. The average annual rainfall is about 49 in., and
about 30 in., or 60 percent. of the annual average, falls during June

BUREAU OF GEOLOGY

Figure 5. Mean monthly rainfall at Tampa, Florida, 1890-1970.

to September (fig. 5). Heavy rainfall is associated also with tropi-
cal depressions and hurricanes, which occur usually from June to
October. The average annual temperature is about 720F (22.0C)
and monthly average temperatures range from about 610F (16.00C)
in January to 820F (27.50C) in August.

TOPOGRAPHY

The Tampa area (fig. 2) is in the sandy and poorly drained
Coastal Lowlands, one of the five topographic divisions of Florida
(Cooke, 1945, and Puri and Vernon, 1964). Along the route of the
canal system from Palm River to Harney Flats, a plain slopes
gently upward from Hillsborough Bay. This plain is a former bay
bottom which was occupied during Pleistocene time by part of an
estuary larger than the present Hillsborough Bay (MacNeil, 1949).
Around Harney Flats, a scarp rims the flat, low-lying swampy plain.
This scarp is the Pamlico Shoreline (MacNeil, 1949) and represents
an advance of the sea to an altitude of about 25 to 35 ft. North and
east of Harney Flats is an upland area which consists of low rolling

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REPORT OF INVESTIGATION NO. 82

hills and features associated with marine terraces, including ponds,
depressions, and swamps.

GROUND-WATER HYDROLOGY
The Tampa area is underlain at depth by several hundred feet
of solution-riddled and fractured limestone and dolomite forma-
tions, which range in age from Eocene to Miocene (Menke and
others, 1961). These formations are, in ascending order, the Lake
City and Avon Park Limestones, Ocala Group, Suwannee and
Tampa Limestones, and the Hawthorn Formation. Along the route
of the canal system, these formations are overlain by as much as
60 ft of unconsolidated sand, silt and sandy clay of Pliocene, Pleis-
tocene, and Holocene ages. In many places these unconsolidated de-
posits are separated from the underlying limestone and dolomite
by thick beds of stiff, green clay. I
These geologic units form a hydrologic system composed of a
shallow water-table aquifer, a confining bed, and the Floridan
aquifer (figs. 6, 7, 8, and 9). In the area through which the canal
system will be built, the saturated parts of the unconsolidated ma-
terials form a shallow water-table aquifer, which has an average
thickness of about 20 ft. Most of the water in this aquifer is de-
rived from local rainfall, and the water table is only a few feet
below land surface (Menke and others, 1961). In this area, the
stiff, green clay has an average thickness of about 10 ft and acts
as a semipermeable confining layer over the limestone and dolomite
formations.
The limestone and dolomite formations constitute an extensive
artesian aquifer system. This aquifer is the principal artesian
aquifer in parts of the southeastern United States (Stringfield,
1966) and is called the Floridan aquifer in Florida (Parker and
others, 1955). The potentiometric surface of the Floridan aquifer
is at or near land surface throughout the area in which the canal
system is being built and is above land surface in some low-lying
areas. The upper part of the Floridan aquifer along the route of
the canal is principally Tampa Limestone.
Water enters the Floridan aquifer in recharge areas and moves
down-gradient to points of discharge. According to Menke and
others (1961), the major part of the recharge to the aquifer in
Hillsborough County is derived locally from leakage through con-
fining beds and sinkholes. During the rainy season, when recharge
to the Floridan aquifer exceeds discharge, water in storage in-

creases, and the potentiometric surface of the aquifer rises. During
the dry season, when discharge exceeds recharge, water in storage
decreases, and the potentiometric surface declines. Thus, the po-,
tentiometric surface is usually highest in September and October.
following the rainy season, and lowest in May and early June just
preceding the rainy season.
The potentiometric surface of the Floridan aquifer slopes down-
ward in the general direction of Hillsborough and McKay Bays, and
the direction of flow in the aquifer is towards the bays, approxi-
mately perpendicular to the lines of equal head. The altitude of
the potentiometric surface of the Floridan aquifer was measured
in wells listed in table 2 and shown on figure 10. Water levels
ranged from near sea level in wells along Hillsborough and McKay
Bays to about 70 ft or more above sea level to the east near Plant
City in May 1972 (fig. 11). The reentrants, or troughs, in the

tWater-level measurement
!Sample for chemical analysis
3Drilled by Corps of Engineers as part of this investigation
-City of Tampa proposed well field in Lower Hillsborough River Detention Area

REPORT OF INVESTIGATION NO. 82

Figure 10. Location of ground-water data-collection sites.

In the Tampa area, the concentrations of dissolved solids, sul-
fate, chloride, and magnesium and the hardness of the water in
the Floridan aquifer generally increase in the direction of water
movement towards Hillsborough and McKay Bays. Also, water at
shallower depths in the aquifer is less mineralized than water at
greater depths. Shattles (1965) indicated that hardness ranges
from less than 200 mg/1 (milligrams per litre) to more than 750
mg/1 near parts of the bays, that concentrations of dissolved solids
range from less than 250 mg/1 to more than 1,000 mg/1, and that
concentrations of chloride range from less than 25 mg/l to more
than 250 mg/l. Water samples were collected at 29 ground-water
sites (table 2 and fig. 10) as part of this investigation, and the
analyses (table 3) are in agreement with the findings of Shattles.

20 BUREAU OF GEOLOGY

TABLE 3.-Analyses of water samples from the Floridan
aquifer in the area of the Tampa Bypass Canal System.

The area in which the canal system is being built (fig. 2) is
drained by several streams (Menke and others, 1961). Palm River
drains 40 mi' in Hillsborough County and empties into McKay Bay.
The stage of the river fluctuates with the tide in McKay Bay. The
river, a continuation of Sixmile Creek, is only 2 mi long and its
average flow probably exceeds 70 cfs, or 45 mgd, at the mouth.
Sixmile Creek rises in the Harney Flats area and flows 7 mi
southward to Palm River. The average discharge at State High-
way 574, 4 mi upstream from McKay Bay, is about 60 cfs or 39 mgd
(U. S. Geol. Survey, 1970), and the discharge on May 22, 1970 dur-
ing the dry season was 31 cfs or 20 mgd. Much of the base flow
of Sixmile Creek comes from springs in the Eureka Springs and
Harney Flats areas.

REPORT OF INVESTIGATION NO. 82

SThe Hillsborough River drains 690 mi". Its headwaters are in
the Green Swamp area of central Florida outside the area of in-
vestigation, and it flows southwestward into Hillsborough Bay at
Tampa. The average discharge at the Hillsborough River State
Park, about 17 mi northeast of Tampa, is about 290 cfs or 187 mgd
(U. S. Geol. Survey, 1970). At the Tampa waterworks dam about
10 mi upstream from the mouth, the flow averages about 670 cfs
or 433 mgd. During the May 1972 dry season, it averaged about
60 cfs or 39 mgd.
Lake Thonotosassa is in the Hillsborough River drainage basin,
and has a surface area of about 830 acres. Baker Creek and Pem-
berton Creek are its principal tributaries, and outflow from the lake
goes into Flint Creek northward to the river. The lake stage aver-
ages about 35 ft above msl.
The Alafia River drains 410 mi". The river flows across the
southern part of the study area and empties into Hillsborough
Bay. Near Lithia Springs, the flow of the Alafia River averages
about 390 cfs or 250 mgd, and at the mouth, the average flow prob-
ably exceeds 460 cfs or 300 mgd.
Numerous springs serve as points of natural discharge from
the Floridan aquifer into the streams. Menke and others (1961)
showed that the area in which water levels in the Floridan aquifer
were above land surface in September and October 1958 coincided
with the locations of many springs and included much of the area
along the stream channels of the Hillsborough and Alafia Rivers,
Sixmile Creek, and the Eureka Springs and Harney Flats areas.
According to Stringfield (1964), springs occur in the river valleys
and low-lying areas where the Hawthorn Formation has been re-4
moved by solution and erosion, and where the Tampa and Suwanneec
Limestones are at or near the land surface.
The discharge from the springs accounts for a large part of the
stream flow in Hillsborough County (Menke and others, 1961).
During low flow, Lettuce Lake in Harney Flats and the springs
in Eureka Springs account for about 80 percent of the discharge
in Sixmile Creek, or about 50 cfs or 32 mgd. Crystal Sprinds, about
20 mi northeast of Tampa, and Sulphur Springs have average dis-e
charges of 62 cfs or 40 mgd hnd 48 cfs or 31 mgd, respectively,
and sustain the flow of the Hillsborough River. About 90 percent
of the flow of Sulphur Springs comes f'rom the upper part of the
Floridan aquifer, or the Tampa and Suwannee Limestones. Lithia

BUREAU OF GEOLOGY

Springs and Buckhorn Spring have average discharges of 45 cfs
or 29 mgd and 12 cfs or 8 mgd, respectively, and contribute to the
flow of the Alafia River.

TEST DRILLING AND AQUIFER TESTS
Two sites (insets A and B, figure 10) were selected by the Corps
of Engineers and the Geological Survey for test drilling and aquifer
testing to determine the hydrologic properties of the Floridan
aquifer along the route of the canal system. One site is along
Sixmile Creek just north of Buffalo Avenue near where the addi-
tional water-level control structure may be located, and the other
site is near Eureka Springs in Harney Flats, where the greatest
effects of the canal system on the aquifer are anticipated.
Two test wells (figs. 12 and 13) were drilled by the Corps of
Engineers to determine the depth, thickness, and productivity of
the water-bearing zones in the upper part of the Floridan aquifer.
One well, P-SMC at the Sixmile Creek site, was 307 ft deep, and
the other well, P-ES at the Eureka Springs site, was 332 ft deep.
Continuous core samples 4 in. in diameter were recovered from
each well, and the geologic formations were identified by William
J. Lang (written commun., 1972). Many fractures and intercon-
nected solution channels were present in the core samples from
the upper part of the Floridan aquifer at both sites, indicating a
highly transmissive aquifer.
At each well, a current meter was used to determine the flow
velocity in the open hole while the well was being pumped at a known
rate of discharge. The cross-sectional area of the hole was calcu-
lated from the diameter indicated by the caliper log; the discharge
was determined from the product of the cross-sectional area and
the flow velocity and plotted as a function of depth (figs. 12 and
13). The greatest increases in discharge Were in the upper part of
the Tampa Limestone, thus showing that this formation is a pro-
ductive water-bearing zone at the two test sites.
At each site, 12 observation wells were drilled to a depth of
about 90 ft extending about 50 ft into the upper part of the Flori-
dan aquifer. The observation wells were spaced about 500, 1,000,
and 2,000 ft from the deep test wells in four directions approxi-
mately normal to each other (Insets A and B, figure 10). The test
wells, P-SMC and P-ES, were filled back to depths of about 100 ft
to the base of the Tampa Limestone and were pumped during the
aquifer tests.

Three aquifer tests were made. The first two tests were made
at the Eureka Springs site during March 30-31 and May 3-6, 1972.
During these tests, lasting 25 and 72 hours, respectively, steady
pumping rates of 525 and 480 gpm were maintained. The third
test was made at the Sixmile Creek test site an May 10-14, 1972,
and a steady pumping rate of 915 gpm was maintained for 96 hours.
The data collected during these tests were analyzed to determine .
the hydrologic properties of the upper part of the Floridan aquifer
and the overlying confining bed. These properties are: transmis- t
sivity, defined as the flow of water in gallons per day through a
vertical strip of the aquifer 1 ft wide extending the full thickness
of the confined aquifer under a unit hydraulic gradient; the coef-
ficient of storage, defined as the volume of water the aquifer re-
leases from storage per unit surface area of the aquifer per unit
change in head; and the leakance, ,which is the ratio of the coef-
ficient of vertical permeability of the confining bed and the thick-
ness of the confining bed. Leakance is a measureof the ability of
leakage to pass through a confining bed vertically.' Leakance can be
defined as the rate of flow that crosses a unit surface area of the
interface between the confining bed and the main aquifer per unit
of head difference between the top and bottom of the confining bed.
Values of transmissivity, coefficient of storage, and leakance
were calculated by fitting the observed values of drawdown of each
observation well to a family of type curves based on the Hantush-
Jacob (1955) leaky-aquifei model. Thus, the discharge was assumed
to be from the release of water stored in the aquifer and to leakage
into the main aquifer from an overlying permeable zone; the re-
lease of stored water from the confining beds during these tests
was assumed to be negligible. In addition, drainage canals were
present at both test sites and acted to some degree as recharge
boundaries, reducing the drawdowns in the observation wells, thus
affecting the shape of the drawdown curves. These effects were
also considered in determining the hydrologic parameters of the,
aquifer. Average values of transmissivity, 400,000 gpd/ft; coeffi-
cient of storage, 5 x 104; and leakance 1 x 10"8 gpd/ft8 were deter- f
mined for the test sites.
HYDROLOGIC EFFECTS OF THE CANAL SYSTEM
EFFECTS OF CANAL SYSTEM AS PRESENTLY DESIGNED
Construction of the canal system will alter the existing hydro-
logic system, causing drawdown (lowering) of the potentiometric
surface of the Floridan aquifer and diversion of flow from other

Figure 14. Hydrogeolic section at Harney Flats showing lowering of the
potentiometric surface of the Floridan aquifer that would result
from the canal system as presently designed.

parts of the hydrologic system. Along much of the route of the
canal system, the upper part of the Floridan aquifer is the Tampa
Limestone, which the test drilling and aquifer tests showed is
highly permeable due to fractures and many interconnected solu-
tion channels. In many places (figs. 7, 8, and 9), the canal system
will cut into the Floridan aquifer or at least cut deeply into the
clay confining layer. Thus, the hydraulic connection between the
canal system and the highly permeable artesian aquifer will be di-
rect at several places, and the potentiometric surface near the canal

REPORT OF INVESTIGATION NO. 82

system will be lowered to the pool level in the canals. Near the canal
system where the potentiometric surface is lowered, a hydraulic
gradient will be established, and water will drain from the aquifer
into the canal system. Because of the high transmissivity of the
aquifer, the potentiometric surface will be lowered for some dis-
tance from the canal system.
The effect of the canal system on the potentiometric surface and
discharge will be greatest in the Eureka Springs and Harney Flats
areas, where the existing potentiometric surface of the Floridan
aquifer is about 8 to 10 ft higher than the pool levels in the canals
will be. In these areas, the canal system will act as a line sink, and,
as presently designed, will lower the potentiometric surface of the
aquifer about 8 to 10 ft in the vicinity of the canal system (fig.
14), and will increase the discharge from the aquifer into the canal
system in these areas.
The other parts of the hydrologic system will respond to the
drawdown and discharge due to the canal system. The drawdown,
in the potentiometric surface will cause an increase in the head
difference between the overlying water-table aquifer and the Flor-
idan aquifer, thus causing an increase in leakage from the water-
table aquifer-into the Floridan aquifer. This increase in leakage will
lower water levels and decrease storage in the water-table aquifer
and tend to dry up swampy areas near parts of the canal system.
The discharge into the canal system from the Floridan aquifer
will cause a decrease in ground-water discharge from the Floridan
aquifer at other parts of the hydrologic system, a reduction in
evapotranspiration, and a reduction of flow in the Hillsborough
River. Spring flow in the Eureka Springs and Harney Flats areas
will be reduced as flow in the Floridan aquifer is diverted into the
canal system, and evapotranspiration losses from these areas may
be reduced slightly due to lowered water-levels in the water-table
aquifer. Flow in the Hillsborough River will be reduced due to a
decrease in the ground-water discharge into the river.
Water quality in the Floridan aquifer will be affected by the
drawdown caused by the canal system. The height of the potentio-
metric surface above sea level and the density difference between
fresh and salt water are two of the principal factors governing the
position of the interface between fresh water and salt water at
depth in the Floridan aquifer. The Ghyben-Herzburg relation
(Walton, 1970) predicts that the distance in feet below sea level
to the interface is equal to 40 times the height of the potentio-
metric surface above sea level if the fresh water was underlain

BUREAU OF GEOLOGY

by sea water. Thus, as the potentiometric surface is lowered in the
Floridan aquifer due to the canal system, the fresh water-salt
water interface eventually may move upward to a new equilibrium
position.
Along the parts of the canal system in the Eureka Springs and
Harney Flats areas where the drawdown could be as great as 10 ft
(fig. 14), the fresh water-salt water interface could move upward
40 times 10 ft, or as much as 400 ft, tending to increase the chloride
and dissolved-solids content of water from the deeper wells in these
areas. The analyses of the water samples from the 29 ground-water
sites (fig. 10 and table 3) document existing conditions and can be
used to determine future changes in water quality in the Floridan
aquifer.

COMPARISON OF THE HYDROLOGIC EFFECTS OF THE
PRESENT AND MODIFIED DESIGNS
A digital model (Pinder and Bredehoeft, 4968, and Trescott,
1973) was used to obtain quantitative estimates of the drawdowni
and discharge due to the canal system and to compare the hydro-
logic effects of the present and modified designs. This model can
accommodate an artesian or water-table aquifer and will compute
values of head and drawdown and a mass balance at selected time
steps at selected nodes.
The area of investigation was divided into a grid. The spacing
between the nodes at the center of each rectangle in the grid
ranged from 2,000 ft along the canal system to 10,000 ft near the
boundaries. The canal system was considered to be hydraulically
connected to the aquifer along C-135 from S-160 to S-159, along
C-136 to S-161, and if built, along C-132 to S-153. Drawdown and
discharge were calculated by treating these segments of the canal
system as a constant-head line sink (fig. 15). Because these canal
segments are not in fact hydraulically connected to the aquifer
along their entire lengths, this calculation procedure should result
in maximum values of drawdown and discharge. The transmissivity
and the storage coefficient of the Floridan aquifer were assumed to
be 400,000 gpd/ft and 5 x 10-4, respectively, and the leakance of
the overlying confiining bed was assumed to be 1 x 10-3 gpd/ft3, all
based on the results from the aquifer tests.
The Hillsborough, Alafia, and Palm Rivers and Hillsborough
and McKay Bays were selected as constant-head boundaries (fig.
15). The configuration of the potentiometric surface (fig. 11) and

REPORT OF INVESTIGATION NO. 82

Figure 15. Parts of the canal system treated as a line sink and boundaries of
digital model.

the numerous springs and marshy lowlands along the river valleys
(Menke and others, 1961) all indicate points of discharge from the
Floridan aquifer into these bodies of water and a high degree of
connection between the aquifer and these water bodies. The eastern
and northeastern boundaries (fig. 15) were far enough from the
canal system so that the predicted drawdown at these boundaries
was negligible.
Steady-state values of drawdown in the potentiometric surface
of the Floridan aquifer and discharge from the aquifer into the
canal were calculated to compare the effects of the canal system
with and without the design modifications. These values will occur
when a new dynamic equilibrium has been established after the
canal system is completed and after the amount of water in storage
in the aquifer is reduced. These values were calculated for a sea-

BUREAU OF GEOLOGY

sonal low water-level period (May 1972), because the effect that
the discharge into the canal system will have on groundwater dis-
charge at other places in the hydrologic system and on flow in the
Hillsborough River will be most critical during the dry season.
During wet periods, the potentiometric surface of the Floridan
aquifer will stand at a higher altitude than shown for the May 1972
period (fig. 11), and, because pool levels in the canal segments will
not change, the drawdown at and discharge into the canal system
will be greater. However, even though the drawdown and discharge
will be greater for wet periods than dry periods, the effect that dis-,
charge into the canal system will havejon ground-water discharge
at other places in the hydrologic system and on flow in the Hills-.
borough River will not be critical during the wet period.

DRAWDOWN
The drawdown of the potentiometric surface of the Floridan
aquifer that would have occurred during the May 1972 seasonal low
water-level period was calculated for four conditions, taking into
account whether the additional control structure at Buffalo Avenue
will be built and whether C-132 (Thonotosassa Canal) will be in-
cluded as part of the final system of canals. The drawdown along
the parts of the canal system treated as a constant-head line sink
was calculated by subtracting the altitude of the proposed pool
levels in the canal system (fig. 3 and 4) from the altitude of the
potentiometric surface for May 1972 along these parts of the canal
system (fig. 10). Drawdown in the irregularly shaped region be-
tween the canal system and the boundaries was calculated using
the digital model.
For the first condition, the canal system consisted of C-135,
C-136, and C-132 without the additional control structure near
Buffalo Avenue. Drawdown was calculated for specific points (fig.
16) and then subtracted from the altitude of the potentiometric
surface for May 1972 (fig. 10) at these same points, giving the
potentiometric surface that would have occurred in May 1972 if the
canal system had been built as presently designed with C-135,
C-136, and C-132 (fig. 17). The hydraulic gradient that would have
been established in the Floridan aquifer extends from the canal
system to some parts of the Hillsborough River, thus indicating
that some flow in the river will be diverted from the river into the
aquifer, discharging into the canal system.
For the other three conditions, only the drawdown was calcu-
lated. Drawdown for the second condition shows the effects of

REPORT OF INVESTIGATION NO. 82

Figure 16. Drawdown that would have occurred during the May 1972 low
water-level period due to the present design of C-135, C-136, and
C-132 if the canal had been built.

C-135, C-136, and C-132 with the control structure at Buffalo
Avenue (fig. 18). Drawdown for the third condition shows the ef-
fects of C-135 and C-136 without either C-132 or the Buffalo
Avenue structure (fig. 19), and drawdown for the fourth condition
shows the effects of C-135 and C-136 without C-132 but with the
Buffalo Avenue structure (fig. 20).
The drawdown will be less with the additional control structure
included in the canal system, because the structure will maintain
the pool level in the Eureka Springs and Harney Flats areas at a
higher level than without the structure. Comparing the first con-
dition to the second condition (figs. 16 and 18) and the third con-
dition to the fourth (figs. 19 and 20), the area within which draw-
down will be equal to or greater than a given value also will be less
with the control structure included.

BUREAU OF GEOLOGY

Figure 17. Potentiometric surface of the Floridan aquifer as it would have
occurred in May 1972 if C-135, C-136, and C-132 had been built.

The areas enclosed by each of the lines of equal drawdown (figs.
15 and 18-20) were measured with a planimeter to compare the ex-
tent of the drawdown due to the present and modified designs. The
area-drawdown relations (figs. 21 and 22) show the effect of the
additional control structure near Buffalo Avenue on drawdown due
to the canal system. For example, for a canal system that consists
of C-135, C-136, and C-132, the area within which drawdown will be
at least 1.0 ft will decrease from 92 mi- to about 51 mi2 or 45 per-
cent, if the additional control structure is built (fig. 21). For the
canal system that consists of C-135 and C-136 without C-132, the
area within which drawdown will be at least 1.0 ft will decrease
from about 48 mi2 to about 24 mi2 or 50 percent, if the additional
control structure is built (fig. 22).

REPORT OF INVESTIGATION NO. 82

Figure 18. Drawdown that would have occurred during the May 1972 low
water-level period due to the modified design of C-135, C-136, and
C-132 with the additional structure near Buffalo Avenue if the
canal had been built.
DISCHARGE
Discharge into the canal system from the Floridan aquifer that
would have occurred during the May 1972 low water-level period if
the canal had been built was also calculated using the digital model
(table 4). This discharge, which would have been made by a de-
crease in the discharge from other parts of the hydrologic system,
was also calculated for four conditions. For a canal system that
consisted of C-135, C-136, and C-132, discharge from the aquifer
into the canal system would have been 22 mgd without the control
structure at Buffalo Avenue and 13 mgd with the control structure.
For the canal system that consisted of only C-135 and C-136, dis-
charge would have been 15 mgd without the control structure and
8 mgd with the control structure. Thus, discharge into the canal

BUREAU OF GEOLOGY

Figure 19. Drawdown that would have occurred during the May 1972 low
water-level period due to the present design of C-135 and C-136
without C-132 if the canal had been built.

Table 4. Discharge into the canal system from the Floridan Aqui-
fer that would have occurred during the May 1972 low
water-level period if the canal had been built.

Figure 20. Drawdown that would have occurred druing the May 1972 low
water-level period due to the modified design of C-135 and C-136
without C-132 but with the additional structure near Buffalo
Avenue, if the canal had been built.

system would be about 40 percent less with the control structure
added to the canal system consisting of C-135, C-136, and C-132
and would be about 45 percent less with the control structure added
to the canal system consisting of C-135 and C-136.

SUMMARY AND CONCLUSIONS
The proposed Tampa Bypass Canal System is designed to divert
flood waters from the Hillsborough River at a point upstream from
areas of flood-plain encroachment in the cities of Tampa and Temple
Terrace through an area east of Tampa into nearby McKay Bay.
The canal system will breach the underlying artesian Floridan
aquifer in several places. Thus, it will cause drainage from the

Figure 21. Area-drawdown relations for C-135, C-136, and C-132 that would
have occurred during the May 1972 low water-level period if the
canal had been built.

aquifer into the canal system and will affect ground-water levels
over a large area.
Test wells were drilled and aquifer tests were made at two sites
along the route of the canal system to determine the geologic and
hydrologic properties of the Floridan aquifer. Continuous core sam-
ples and current-meter tests showed that the upper part of the
Florida aquifer contains many fractures and interconnected solu-
tion channels and is a productive water-bearing zone at both sites.
Three aquifer tests were made, and average values of tranmis-
sivity, the storage coefficient, and leakance were determined to be
400,000 gpd/ft, 5 x 10-4, and 1 x 10-3 gpd/ft3, respectively.
Construction of the canal system will alter the existing hydro-
logic system, lowering the potentiometric surface of the Floridan

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REPORT OF INVESTIGATION NO. 82

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PRESENT DESIGN OF C-135 and C-136 (10- FOOT POOL)

O MODIFIED DESIGN WITH THE ADDITIONAL STRUCTURE NEAR BUFFALO AVENUE
(10- AND 15-FOOT POOLS)
Figure 22. Area-drawdown relations for C-135 and C-136 that would have
occurred during the May 1972 low water-level period if the canal
had been built.

aquifer and diverting flow from other parts of the hydrologic sys-
tem. The greatest effect on the potentiometric surface will be in
the Eureka Springs and Harney Flats areas, where the canal sys-
tem will act as a line sink, and, as presently designed, will lower
the potentiometric surface of the aquifer as much as 10 ft or more
in the vicinity of parts of the canal system, increasing the dis-
charge into the canal system from the aquifer in these areas.
The other parts of the hydrologic system will repond to the
drawdown and discharge due to the canal system. An increase
in leakage from the water-table aquifer'will lower water levels
in the water-table aquifer and tend to dry up swampy areas near

BUREAU OF GEOLOGY

parts of the canal system. The discharge into the canal system
from the Floridan aquifer will cause a decrease in discharge from
other parts of the hydrologic system. Thus, spring flow in the
Eureka Springs and Harney Flats areas will be reduced, and flow
in the Hillsborough River will be reduced due to a decrease in
ground-water discharge and some diversion of flow from the river.
As another consequence, the fresh water-salt water interface in
the Floridan aquifer will move upward to a new equilibrium posi-
tion. tending to increase the chloride and dissolved-solids concen-
tration of water in the deeper wells in the Eureka Springs and
Harney Flats areas where the drawdown will be the greatest.
As part of this study, the effect of adding another water-level
control structure near Buffalo Avenue was investigated. This struc-
ture would maintain the pool level in the Eureka Springs and Har-
ney Flats areas at a 5 ft higher level than without the structure.
This would result in less drawdown in the potentiometric surface
of the Floridan aquifer and less discharge into the canal system
from the aquifer in these areas, reducing the hydrologic effects.
A digital model was used to obtain quantitative estimates of the
drawdown and discharge and to compare the effects of the present
and modified designs. The estimates represent a maximum condi-
tion because of assumptions used in the model. Certain segments of
the canal system were assumed to have direct hydraulic connection
with the Floridan aquifer along their entire length, an assumption
which is valid for only part of their length. Also, calculations were
made for drawdown and discharge that would have occurred during
the May 1972 seasonal low-water period due to the present and
modified designs of the canal system. The calculations were made
for a seasonal low-water period, because the effect that the dis-
charge into the canal system will have on ground-water discharge
at other places in the hydrologic system and on flow in the Hills-
borough River will be most critical during the dry season. Four
conditions were investigated, taking into account whether the ad-
ditional control structure near Buffalo Avenue will be built and
whether C-132 (Thonotosassa Canal) will be included in the final
system of canals. The canal system, as presently designed, will
lower the potentiometric surface of the Floridan aquifer by 1 ft
or more over an area of about 92 mi2 with C-182 (Thonotosassa
Canal) included in the system and about 48 mi" without C-132;
discharge from the aquifer into the canal system will be about
22 mgd with C-132 and 15 mgd without C-182. The discharge into
the canal system will cause a decrease in the present (pre-canal sys-

REPORT OF INVESTIGATION NO. 82

ter) discharge from the Floridan aquifer to other parts of the
hydrologic system. If another control structure is added to the canal
system near Buffalo Avenue, the area of drawdown of the potentio-
metric surface will be less by 45 percent and the discharge less by
40 percent with C-182 included in the system; without 0-132, the
area drawdown will be less by 50 percent and the discharge less by
45 percent.
Adding another control structure near Buffalo Avenue will re-
duce appreciably the drawdown and discharge due to the canal sys-
tem. As a result, downward leakage from the water-table aquifer
will be less, and the lowering of water levels in the water-table
aquifer also will be less. Also, the effects on spring flow in the
Eureka Springs and Harney Flats areas and on flow in the Hills-
borough River will be less. In addition the upward movement of
the fresh water-salt water interface in the Floridan aquifer will be
less, reducing the effects of the canal system on water quality.
Therefore, the additional control structure near Buffalo Avenue
will reduce the effects of the canal system on the hydrology of the
area.